In recent years, in the process of forming high quality thin films and in dry etching process for forming fine patterns, it is becoming more and more important that the process atmosphere be ultra high clean, that is, a technique for supplying an ultra high pure gas to a process apparatus.
For example, regarding a semiconductor device, the size of unit elements are becoming smaller and smaller year by year in order to increase the degree of integration of integrated circuits, and research and development for realizing semiconductor devices having a size of from 1 mm to a submicron size, or even a size of not more than 0.5 mm, is carried out actively. The production of such a semiconductor device is achieved by repeating such processes as forming thin films or etching the thin films to a predetermined circuit pattern. It has been becoming usual that such processes are carried out by putting silicon wafers into a process chamber and under a reduced pressure atmosphere to which a predetermined gas is introduced. The purpose of the reduced pressure state is to increase the mean free path of the gas molecules and to control the gas phase reaction for etching or packing through-holes or contact holes having a high aspect ratio.
When an impurity is mixed with the reaction atmosphere of these processes, such problems occur that the quality of thin films deteriorates, that the process precision in etching is not sufficient, that the selectivity deteriorates, and that the adhesivity between the thin films becomes insufficient. In order to produce integrated circuits having patterns of a submicron or a lower submicron size on a large diameter wafer at a high density and high production yield, the reaction atmosphere contributing to the film formation and etching must be controlled completely. This is why a technique for supplying an ultra high pure gas is important.
As gasses to be used in a semiconductor producing apparatus, there are general gasses which are relatively stable, such as N.sub.2, Ar, He, O.sub.2, H.sub.2, and special material gasses which are highly toxic, spontaneously combustible and corrosive, such as AsH.sub.3, PH.sub.3, SiH.sub.4, Si.sub.3 H.sub.5, HCl, NH.sub.3, Cl.sub.2, CF.sub.4, SF.sub.6, NF.sub.6, WF.sub.6 and F.sub.2.
Since the general gasses can be handled relatively easily, they are in most cases fed under pressure from a purification apparatus directly to a semiconductor producing apparatus, and by developing and improving containers, purification apparatuses and piping materials, it has become possible to supply ultra high pure gasses (Tadahiro Ohmi, "Challenge against ppt--Gas piping system for gasses for semiconductors challenging ppt impurity concentration", Nikkei Micro Device, July 1987, pp. 98-119).
On the other hand, the special gasses require sufficient attention in handling Since the amount of the special gasses used is small, they are in most cases contained in cylinders and fed under pressure through a cylinder cabinet piping apparatus to the semiconductor producing apparatus.
In connection to this, in the case of the special material gasses, they are usually diluted with a diluting gas (balance gas) to a low concentration to obtain process gasses, and they are contained in cylinders. A process gas supplying apparatus is constructed in such a manner that a process gas is supplied from a cylinder to the process apparatus. Therefore, the cylinder is changed with a new one, as frequently as once in a month or a week, so that even when a cylinder improved in respect to the inner surface contamination is used, the inside of the gas supplying piping system is contaminated with the atmosphere penetrating into it. Moreover, although a technique for filling an ultra high pure gas is established, the purity of the gas contained in the cylinder becomes lower than that before the filling. Furthermore, the changing of the cylinder requires man power, so that the production cost increases.
When the process gas to be supplied to the process apparatus is contaminated, the processes are influenced profoundly. For example, in an Al-film formation technique (T. Ohmi. H. Kuwabara, T. Shibata and T. Kiyota, "RF DC coupled mode bias sputtering for ULSI metallization", S. Broydo and C.M. Osburn, Ed., "ULSI Science and Technology/1987", The Electrochemical Society Inc., Philadelphia 1987, Proc. Vol. 87-11, pp. 574-592, and Tadahiro Ohmi, "Al film formation conditions completely eliminating impurities and preventing generation of hillock are found", Nikkei Micro Device, October, 1987, pp. 109-111), when water is contained at a concentration of 10 ppb in an Ar sputter atmosphere, the morphology of Al film surface deteriorates. In such a state, it is impossible to optimize the film formation parameters of Al whose resistance is the same as that of bulk Al and which is free from generating hillock at heat-treatment. Moreover, when this film formation technique is applied to the formation of Si films, even when other film formation conditions are kept the same, only amorphous films are obtained if the process atmosphere is contaminated with gasses released from the inner surface of the chamber (T. Ohmi, T. Ichikawa, T. Shibata, K. Matsudo and H. Iwabuchi, "In Situ Substrate-Surface Cleaning for Very Low Temperature Silicon Epitaxy by Low-Kinetic-Energy particle Bombardment", Appl. Phys. Lett., 53, 4 July (1988), and T. Ohmi, T. Ichikawa, T. Shibata, K. Matsudo and H. Iwabuchi, "Low-Temperature Silicon Epitaxy by Low-Energy Bias-Sputtering", Appl. Phys. Lett., Aug. 1, (1988)).
Furthermore, when Si thin films are formed by means of reduced pressure CVD, if the water content exceeds 10 ppb, neither selective growth nor epitaxial growth takes place (Junichi Murota, Naoto Nakamura, Manabu Kato, Nobuo Mikoshiba and Tadahiro Ohmi, "Ultra clean CVD technique having high selectivity", Abstracts of Oral Presentations [Process Technology for Higher Performance III], The 6th Ultra LSI Ultra Clean Technology Symposium, January 1988, pp. 215-226).
Furthermore, the conventional process gas supplying apparatus employees such methods that the gas flow rate is controlled by selecting the number of capillaries through which the gas flows by changing some of them, or that the flow rates of two gasses to be mixed with each other are controlled by means of two float-type flowmeters or a mass flow controller. However, the dilution ratio in these methods is as small as one by several hundreds, and the gas contacting part is in most cases too dirty to supply an ultra high pure gas. For example, when a gas of a concentration of from several ppm to several tens of ppm is to be supplied to the process apparatus, if a 100% source gas is supplied from a cylinder, the source gas has to be diluted to a degree of one by ten thousand or one by one hundred thousand with a balance gas.
The present invention has been achieved in view of the above situation, and an object of the invention is to realize a gas supplying system which easily supplies a process gas without being influenced by atmosphere contamination and which is almost free from releasing gasses having undesirable influences on the Processes, such as water, organic materials and the like, and to provide a highly clean and high performance process gas supplying apparatus.